A “forward voltage” as used herein refers to the voltage drop across a component, or a plurality of components, of a constant current LED pulsing drive circuit. For example, the forward voltage across an LED is the voltage drop across the LED. Similarly, the forward voltage across a series of LEDs is the voltage drop across the series of LEDs.
A “bias voltage” as used herein refers to a voltage having a value comprised of at least two components: a first component associated with the forward voltage across one or more LEDs, and a second component associated with the voltage across the other components in series with the one or more LEDs.
An LED will emit light when at least a minimum voltage (hereinafter “Vmin”) is supplied to appropriate leads of the LED. Most LEDs are produced in batches. The Vmin of each LED in a batch will be between a range of forward voltages. The Vmin of each LED may change over the life of the LED but will remain between the range of forward voltages. LEDs are manufactured, however, in batches having a wide range of forward voltages. For exemplary purposes only, an LED batch may have a range of forward voltages between 1.7 volts to 3.4 volts.
Referring to FIG. 1A (Prior Art), an exemplary constant current LED pulsing drive circuit is shown having a series of four LEDs 1 and a fixed voltage supply 2. Previously, if the series of four LEDs 1 were from the example batch above (i.e., range of forward voltages being 1.7 volts to 3.4 volts), then each of the series of four LEDs 1 would be assumed to have a forward voltage of 3.4 volts (i.e., the worst case scenario). Hence, the first component of the bias voltage supplied through the series of four LEDs 1 would be 13.6 volts. This is inefficient, however, because the Vmin of each of the series of four LEDs 1 may be less than 3.4 volts (in fact, the Vmin could be as small as half that amount). When a supplied voltage exceeds the Vmin, the additional voltage supplied may be converted into heat, increasing wear and power consumption, and thus require thermal management parts.
Real-time closed loop control has been proposed to address these inefficiencies in operating LEDs having a wide range of forward voltages. In previous schemes, a current flowing through an LED is detected, and a bias voltage supplied to the LED is adjusted in response to the detected current automatically and at all times current flows through the circuit. For example, two previous schemes are illustrated in FIG. 1B (Prior Art). In each of these schemes, the bias voltage supplied by the power stage 3 is continually adjusted to provide a bias voltage having a first component equal to Vmin.
While these previous schemes improve efficiency in some circumstances, the adjustment of the bias voltage to a stable state is limited by the electric circuit response time (i.e., the time it takes from bias voltage adjustment start to output stabilized). Generally, the power stage of the previous schemes determines the response time. The response time for previously proposed schemes has been as fast as approximately 1 millisecond. An exemplary power stage is shown in FIG. 1C (Prior Art), and a plot of voltage versus time is shown in FIG. 1D (Prior Art). The plot shown in FIG. 1D is from a simulation of real-time closed loop control using the power stage shown in FIG. 1C. In the simulation, the bias voltage was stepped up from 24 volts to 28 volts and stepped down from 28 volts to 24 volts. The response time 4 associated with the step up in voltage was observed to be 0.931 milliseconds. The response time 5 associated with the step down in voltage was observed to be 1 millisecond.
In some applications, it is desirable to supply pulses of current to the LED in order to cause the LED to flash on and off. In many such applications, the length of each pulse may be very short (e.g., less than 300 microseconds). In applications having pulses shorter than the response time of the circuit (e.g., as short as 300 microseconds or less), the previously proposed schemes do not work because the response time is too long. That is, each pulse of current terminates before the bias voltage supplied to the LED can be changed and stabilized. Attempting to implement the previously proposed schemes in applications with short pulse width (e.g., less than 300 microseconds) causes unexpected changes in voltage and/or instability in the system.